NCERT Solutions Curiosity Chapter 9 .1.1 Digestion in human beings — In-text Questions
Book page 1249 Updated on2026-09-05
Q1.
What causes the change of colour in test tube A?
Answer
The starch of the boiled rice causes it. In Grade 6 we learnt that iodine gives a blue-black colour when it reacts with starch. The rice in test tube A was never chewed, so no saliva ever touched it and its starch is still whole. Iodine finds that starch and turns blue-black.
Test tube A: boiled rice (starch present) + iodine → blue-black
Test tube B: chewed rice (starch broken to sugar) + iodine → no change / very light colour
Why it happens: saliva acts only on the food it is mixed with. Tube A is the control — it shows what the rice looks like when saliva has not acted. Comparing it with tube B is what proves that saliva digested the starch.
Q2.
If the colour still appears in test tube B, what changes would you make in the activity to explore it further? Would the colour change if chewing time is increased?
Answer
If tube B still shows a blue-black colour, some starch is left undigested. Change one thing at a time and repeat:
Chew for longer — 90 seconds or 2 minutes instead of 30–60 seconds, so saliva gets more time to act.
Chew more thoroughly, so that the rice is broken into finer pieces and more of it is exposed to saliva.
Take a smaller amount of rice in the mouth, so that the saliva available is enough for it.
Keep the mixture standing for some minutes before adding iodine, giving the saliva already mixed in more time to work.
Add fewer, counted drops of iodine (the same number in both tubes) so that the comparison is fair.
Yes — increasing the chewing time does change the colour. The longer you chew, the more starch is turned into sugar, so the blue-black colour becomes lighter and lighter and finally does not appear at all.
Check it yourself: take four tubes with rice chewed for 15 s, 30 s, 60 s and 120 s. Line them up after adding iodine — you will see a clear ladder of colour from dark to almost none.
Q3.
But how does the food move down?
Answer
The food does not fall down by its own weight — it is pushed. The walls of the food pipe gently contract and relax in a wave-like motion, and this travelling wave squeezes the food forward into the stomach.
Fig. 9.2 in the book: the wall squeezes behind the food and relaxes in front of it, so the food is pushed towards the stomach.
Why it happens: the food pipe has muscles in its wall. When the muscle behind the lump of food tightens and the muscle in front of it loosens, the food has only one way to go — forward. This same wave-like movement takes place throughout the alimentary canal, which is why food keeps moving even when you lie down, and why an astronaut can swallow in space.